It is known to use a mass filter in a mass spectrometer to select parent ions having a certain mass to charge ratio. The selected parent ions may then, for example, be fragmented in a collision or fragmentation cell and the resulting fragment ions can then be mass analysed by a mass analyser. The mass filter most commonly used to select parent ions having a certain mass to charge ratio is a quadrupole rod set mass analyser. However, other types of mass filters are known including Wien filters and Bradbury-Nielsen ion gates.
A Wien filter operates by passing a beam of ions through crossed electric and magnetic fields. Ions having a mass m, charge q and velocity v will pass undeviated through the filter if:Eq=Bqvwhere E and B are the electric and magnetic field strengths respectively. Accordingly, if all the ions in an ion beam have essentially the same energy, then only ions having a particular mass to charge ratio will have the required velocity to pass through the filter undeflected. However, disadvantageously, the resolution of a Wien filter is dependent upon the absolute magnitude of the crossed electric and magnetic fields experienced by the ion beam. Since large magnetic fields require very large electromagnets then the ultimate resolution of a mass spectrometer incorporating a Wien filter is, in practice, fairly restricted, particularly at higher mass to charge ratios. A maximum mass to charge ratio resolution of approximately 400 is common for known mass spectrometers which incorporate a Wien filter. The mass to charge ratio resolution R may be defined as:
  R  =      m          Δ      ⁢                          ⁢      m      where Δm is a mass to charge ratio window transmitted at a mass to charge ratio m. The large physical size of the various components necessary to form a Wien filter in addition to its limited resolution has relegated its use to certain specialised areas such as atomic physics and ion implantation.
Quadrupole rod set mass filters, by contrast, are relatively compact and are commonly used in commercial mass spectrometers. A quadruple rod set mass filter comprises two electrically connected pairs of cylindrical rod electrodes to which both RF and DC voltages are applied. For a given RF frequency and at appropriate setting of the RF and DC voltages, only ions having a very limited range of mass to charge ratios will have stable trajectories through the quadrupole rod set mass filter. Accordingly, only ions having a certain mass to charge ratio will be transmitted by the quadrupole rod set mass filter. Ions having other mass to charge ratios will have unstable trajectories within the rod set mass filter and will collide with the cylindrical rod electrodes and hence become lost to the system.
Quadrupole rod set mass filters are particularly advantageous in that they can have resolutions of several thousand. However, disadvantageously, in order to operate effectively quadrupole rod set mass filters require that the ion beam which is to be mass filtered should have a relatively low energy. Quadrupole rod set mass filters also have a relatively limited mass to charge ratio range and must be manufactured and constructed to very high tolerances. Furthermore, quadrupole rod set mass filters suffer from the problem that the particular RF power supplies which are used with such mass filters are physically relatively large. This is particularly problematic when seeking to provide a compact bench-top mass spectrometer.
A Bradbury-Nielsen ion gate can be used as a mass filter. The ion gate may, for example, be provided in a flight region of a mass spectrometer wherein ions take different times to traverse the flight region depending upon their mass to charge ratio. The ion gate may be arranged so as only to allow ions having a relatively small range of mass to charge ratios to be transmitted. This is achieved by rapidly opening and then closing the electrostatic ion gate at a time equal to the arrival time of ions having mass to charge ratios of interest.
Bradbury-Nielsen ion gates comprise parallel electrodes between which an ion beam is directed. An electric field is created in use between the electrodes of the ion gate. The electric field, when created, is sufficient to deflect the beam of ions away from their original path and hence the ion gate can be considered to be closed or otherwise to have a transmission of 0% when an electric field is created. In order to open the gate or otherwise to provide a transmission of 100%, the electric field maintained between the electrodes is switched OFF or is otherwise reduced to zero for a very short period of time. This enables ions having a desired mass to charge ratio to pass through the ion gate without being deflected by an electric field. As soon as ions having the desired mass to charge ratio have been transmitted, the electric field is restored and ions subsequently arriving at the ion gate are deflected away from their original path.
In theory, the mass to charge ratio range of a Bradbury-Nielsen ion gate is unlimited. However, in practice, the resolution achievable with a Bradbury-Nielsen ion gate tends to be disappointingly low e.g. approximately 20-50 for dual-electrode arrangements and of the order of 100-200 for multi-electrode arrangements. The placement of electrodes very close to the path of an ion beam also tends to lead to a loss in ion transmission even when the ion gate is not being used as a mass filter since some ions will still tend to strike the electrodes. As a result, Bradbury-Nielsen ion gates are not commonly used as mass filters in commercial mass spectrometers.
Time of flight mass filters are also known which, like Wien filters, transmit all ions having a certain specific velocity. However, disadvantageously, ions having different mass to charge ratios but which happen to have substantially the same velocity will be simultaneously transmitted by such mass filters. This can be problematic in a number of different scenarios. For example, if a precursor or parent ion fragments (either spontaneously due to Post Source Decay or due to Collision Induced Dissociation in a collision or fragmentation cell), the resulting fragment ions will retain essentially the same velocity as the corresponding precursor or parent ion had. Accordingly, if a precursor or parent ion fragments upstream of a time of flight mass filter, then fragment ions together with corresponding unfragmented parent ions will be simultaneously transmitted by the time of flight mass filter. Accordingly, the time of flight mass filter will transmit ions having substantially different mass to charge ratios at substantially the same time.
It is therefore apparent that there are a number of problems associated with conventional mass filters.
It is therefore desired to provide an improved mass filter.